The US defence secretary has forced out the country's commander in Afghanistan, saying the battle against the Taleban needs "new thinking".
Robert Gates confirmed Gen David McKiernan would effectively be sacked less than a year after taking command.
He will be replaced by Gen Stanley McChrystal, who is seen as having a better understanding of the conflict.
The change comes as the US boosts troops numbers in Afghanistan and prepares for a change in strategy.
Gen McKiernan's time as US commander in Afghanistan has coincided with a surge in violence.
His successor currently serves as the director of US Joint Chiefs of Staff, and was previously a director of special operations forces.
Jonathan Beale BBC News, Washington
Gen McChrystal was in charge of Joint Special Operations in Iraq. His forces were involved in the capture of Saddam Hussein and the killing of al-Qaeda's leader in Iraq - Abu Musab al-Zarqawi.
Robert Gates has refused to explain why he lost faith in Gen McKiernan. But both he and President Obama have often repeated that the war in Afghanistan will not be won with military strength alone.
The inference is that Gen McKiernan was seen as too conventional a military commander. Brilliant at organising a ground war - as he did in Iraq - but less equipped for the complexities of Afghanistan.
Gen McChrystal is reported to have adopted an approach of "collaborative warfare" - relying on communication intercepts and human intelligence as well as military force.
Announcing the removal of Gen McKiernan from his role, Mr Gates said new military leadership was needed to go along with a new strategy and a new ambassador.
"This is the right time to make the change," he said.
"Our mission there requires new thinking and new approaches from our military leaders."
He said the decision was in the best interest of US national security and the success of the Afghanistan mission.
It was made after consulting the chairman of the Joint Chiefs of Staff, Admiral Mike Mullen, and the commander of the US Central Command, Gen David Petraeus.
The change also had the approval of President Barack Obama.
is a specialist in the kind of counter-insurgency strategy the Obama administration plans to implement in Afghanistan.
The change comes as President Obama's administration prepares to send thousands of extra troops to Afghanistan, and amid pressure on international forces to reduce the numbers of civilians killed by coalition air strikes.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.